Dissection of the mutation accumulation process during bacterial range expansions.

Area of Science:

• Evolutionary biology
• Microbial ecology
• Genomics

Background:

• Spatial expansion dynamics in bacteria differ from well-mixed conditions.
• Deleterious mutations can accumulate on expansion fronts, affecting adaptive evolution.

Purpose of the Study:

• To investigate the genomic evolution of bacteria during spatial expansion.
• To understand the interplay between adaptive and non-adaptive evolutionary processes.

Main Methods:

• Whole genome sequencing of 10 mutator Escherichia coli lines over 39 days (approx. 1650 generations).
• Elastic net regression to infer mutation effects on colony growth.

Main Results:

• Colony size decreased in all lines by the experiment's end.
• Mutations accumulated at a constant rate.
• Beneficial mutations primarily occurred early, influencing non-linear colony size changes.
• Beneficial mutations enriched in genes for transport and membrane structure.
• Deleterious mutations showed no specific biological process enrichment.

Conclusions:

• Beneficial mutations target specific functions, particularly inter/extramembrane processes.
• Deleterious mutations are randomly distributed across the genome.
• Genetic drift and mutation availability/depletion influence bacterial fitness during range expansion.